The present invention relates to a process for the effective regeneration of cyanide through the oxidation of thiocyanate. More particularly, the present invention relates to the use of ozone to oxidize thiocyanate and regenerate free cyanide for subsequent use. Such a process is particularly useful in the treatment of CN tailings.
Cyanide is used in a variety of industrial processes that are carried out in aqueous media, such as the recovery of gold from ore, electroplating, and the conversion of domestic coal to coke, gasoline and gaseous fuels. Many gold mining operations use cyanide solutions to leach gold from the ore. Cyanide consumption is one of the main components of the total operating cost of a plant. However, most of the cyanide added to a slurry is actually wasted. According to the theoretical stoichiometry, to dissolve the gold contained in a typical ore, just 3 to 4 grams of cyanide per ton should be consumed. In practice, typical consumptions range from about 300 g/t to some 2000 g/t or even more. The extra cyanide consumption is partially accounted for by the formation of volatile HCN and by oxidation to cyanate, but the biggest loss occurs through the formation of cyanide complexes (copper, iron and zinc) and of thiocyanate. According to some reports, cyanide loss through thiocyanate may account for up to 50% of the total cyanide consumption. In fact, in all of the aforementioned industrial processes, a wastewater is generally created which contains a large concentration of thiocyanate.
Thiocyanate concentration in mill effluent varies widely depending on the type of ore and operating conditions. A normal range would be 40 mg/l to 600 mg/l, but it may reach up to nearly 2000 mg/l in the barren bleed of certain Merrill-Crowe operations. The exact mechanism of thiocyanate formation during cyanidation is not clear. It is believed that thiocyanate results from the reaction of the cyanide ion or its metallic complexes with sulfur atoms originated from the attack of alkalies on reactive sulfides such as pyrrhotite. Sulfur atoms can also form by the oxidation of sulfide ions released by dissolution of sulfide minerals. Thiosulfate and polythionate species may also contribute to thiocyanate formation.
Thiocyanate is considered to be non-toxic and its concentration in mining effluent is not regulated at the present time. However, it is known that ultraviolet light decomposes thiocyanate to form cyanide, it is then possible that sunlight may liberate cyanide levels toxic to aquatic life from effluent rich in thiocyanate. In view of these considerations, it is not unlikely that in the future some limit may be imposed on the concentration of thiocyanate in effluent.
Among the leading processes currently being applied to the detoxification of a cyanidation effluent are processes based on the oxidation of cyanide with hydrogen peroxide or with SO.sub.2 -air mixtures. See, for example, U.S. Pat. No. 5,178,775. Other typical processes for removing cyanide from aqueous streams are described in U.S. Pat. Nos. 4,537,686; 5,015,396; and 5,290,455. None of the current oxidants used, however, are strong enough to decompose thiocyanate.
In order to destroy thiocyanate, more powerful reagents such as ozone or chlorine are required. Chlorine, however, may produce toxic derivatives and its use in the mining industry as well as other industries is questionable. On the other hand ozone is a less problematic reagent. At present, ozone is widely applied in water treatment and it is increasingly used instead of chlorine as a bleaching agent in the pulp and paper industry. The use of ozone to destroy cyanide in a mining effluent has received some attention. See, for example, M. D. Durol and T. E. Holden, "The Effect of Copper and Iron Complexation on Removal of Cyanide by Ozone." Ind. Eng. Chem. Res., vol. 27, No. 7. 1988, pp. 1157-1162, and Treatment of Acid Ming Drainage by Ozone Oxidation, Department of Applied Science, Brookhaven National Laboratory, Associated Universities, Inc., prepared for the Environmental Protection Agency, December, 1970. However, the economics of the process has not been demonstrated.
Little information has been published regarding the reaction of thiocyanate with ozone. It is known that the reaction occurs in two steps, the first one involving the oxidation of thiocyanate to cyanide, with the second involving further oxidation to cyanate, according to the following equations. EQU SCN.sup.- +2O.sub.3 +2OH.sup.- .fwdarw.CN.sup.- +SO.sub.4.sup.2- +H.sub.2 O+3/2O.sub.2 ( 1) EQU CN.sup.- +O.sub.3 .fwdarw.CNO.sup.- +O.sub.2 ( 2)
Both reactions are fast and are probably mass transfer controlled. According to these equations, regeneration of cyanide from thiocyanate would be difficult due to the subsequent oxidation of the cyanide formed. Nevertheless, the ability to successfully regenerate cyanide from thiocyanate would be of great value to the industry as cyanide regeneration seems more attractive than straight thiocyanate destruction. Cyanide regeneration would require less oxidizing agent and it would also reduce overall cyanide consumption.
Accordingly, it is an object of the present invention to provide a process for the controlled oxidation of thiocyanate to regenerate cyanide.
Another object of the present invention is to provide a process for the effective and efficient oxidation of thiocyanate to recover cyanide from cyanidation effluents.
Another object of the present invention is to successfully apply such a recovery process to effluents created in gold mining, electroplating and in coal conversion.
These and other objects of the present invention will become apparent upon a review of the following specification and the claims appended thereto.